1. Field of the Invention
The invention relates to the field of optical imaging and in particular to the use of optical polarizations to create tomographic images.
2. Description of the Prior Art
The demand for noninvasive optical images of biological tissue has led to the development of several techniques to circumvent the common problem of scattering in turbid media. Such techniques include diffusing wave spectroscopy A. Yodh, et al., "Spectroscopy and Imaging with Diffuse Light," Physics Today 48, 34 (1995); F. Liu, et al., "Transmitted Photon Intensity through Biological Tissues within Various Time Windows," Opt. Lett. 19, 740 (1994); W. Denk, et al., Handbook of Biological Confocal Microscopy, edited by J. B. Pawley (Plenum New York 1995) at p445; L. Wang, et al. "Continuous-Wave Ultrasonic Modulation of Scattered Laser Lidght to Image Objects in Turbid Media," Opt. Lett. 20, 629 (1995); M. Kempe, et al., "Acousto-optic tomography with Multiple Scattered Light," J. Opt. Soc. Am. A14 (1997); and D. Huang, et al. "Optical Coherence Tomography", Science 254, 1178 (1991); and A. F. Fercher, "Optical Coherence Tomography", J. Biomed. Opt. 1, 157 (1996).
Optical coherence tomography employs the partial coherence properties of a light source to image structures with high resolution, namely, in the range of one to fifteen microns, in turbid media such as biological tissue. The sample under study is positioned in one arm or sample arm of a two beam interferometer. The interference fringes are formed when the optical path length of the signal back scattered from the sample matches that from the reference arm to within the coherence length of the source light. The optical path length in the reference arm acts as a detection gate to thereby select only light back scattered from the sample that has traveled the same optical path length. By laterally and longitudinally scanning the sample, two dimensional optical coherence tomographic images are constructed that map the amplitude of light back scattered from the sample. Lateral resolution is determined by the spot size of the beam focus of incoming light and the collection aperture. Longitudinal resolution is determined by the coherence length of the source light.
Optical coherence-domain reflectometry is further described by Sorin, et al. "Polarization Independent Optical Coherence-Domain Reflectometry," U.S. Pat. No. 5,202,745 (1993). Sorin describes the use of an incoherent light source in an interferometer in which the reference mirror is moved at a controlled velocity to produce a Doppler shift in the reference signal in the interferometer. The phase of the reference signal is also modulated by piezo-electric transducer to cause a further shift in the referenced signal frequency. The interference signals are detected and measured by a polarization diversity receiver. A linear polarizer and reference signal arm is adjusted to produce equal reference signal powers in each arm of the polarization diversity receiver in the absence of reflection signal from the test arm. The measured reflectometry signal is substantially independent of the state of polarization and the reflected signal from the device under test. The system is generally used to test the optical properties of optical fiber systems. Polarization stability can be a problem in such testing situations. Sorin is directed to a methodology to overcome the problem of polarization variations and distortions in optical fibers and system components. Sorin thus describes the system in which the polarization information returned from the sample or test object is treated as noise and is effectively disregarded.
Prior art examples of the use of optical coherence domain reflectometry is described in Swanson, et al. "Method and Apparatus for Performing Optical Measurements," U.S. Pat. No. 5,459,570 (1995). The use of the method for performing optical imaging and sample using longitudinal scanning and positioning of a sample by varying the relative optical path lengths leading to the sample into a reference reflector or by varying the optical characteristic of the output from the optical source is shown and described in Swanson, et al., "Method and Apparatus for Optical Imaging with Means for Controlling the Longitudinal Range of the Sample," U.S. Pat. No. 5,321,501 (1994).
Swanson '570 and '601 describes inter alia a system in which birefringence information may be obtained by polarizing the optical radiation used, by suitably modifying the polarization in the sample and reference paths, and by dividing the output into an orthogonal polarization outputs which are separately demodulated before processing. In particular, in FIG. 6 in Swanson '570 orthogonal polarized back scattered interference fringes are provided to two separate detectors 42C and 42D into a beam splitter in the return arm of an interferometer using a coherent light source 12 as the interferometer source signal. The interferometric signals detected by detectors 42C and D are separately processed in demodulators 46 and A-to-D converters 50 to produce two interferrometric signals, a horizontal amplitude component 11 and a vertical amplitude component 12. These signals are applied to computer 52 and used to determine the round trip birefringence retardation in the sample like path. Swanson does not in fact measure the phases from two detector outputs. The relative phase information is computed from measured amplitudes of the two orthogonal components under a set of special assumptions. In order to obtain computed phase information from the signals in Swanson, the optical birefringence axes in the sample must be constant.
Demodulator 46 for each of the signals is detailed further in FIG. 2. Demodulator 46 is comprised of a band pass filter 78 centered around the modulation frequency of the combined output signal and an envelope detector. The filter assures that only the signal of interest is looked at and removes noise from the output. The filter signal is applied to the envelope detector. The envelope detector is comprised of a rectifier 82 and a subsequent low pass filter 84. The rectifier output is proportional to the square of the sample reflectivity. The low pass filter removes any high frequency components from what is basically a base band signal. Demodulator 46 also includes a logarithmic amplifier 86 for dynamic range compression.
The effect of rectification of the signal is graphically illustrated by a comparison of FIGS. 5A and 5B. FIG. 5A illustrates the output waveform modulation frequency on which the envelope is superimposed. FIG. 5B is the wave form of FIG. 5A after demodulation by demodulator 46. Thus, it is clear from Swanson that the information contained within the fringes or oscillations of FIG. 5A are lost in the demodulation. As will be described below, it is this information in the undemodulated signal of Swanson that provides a basis when used according to the invention to develop a two dimensional birefringence imaging signal, and more generally create a polarization sensitive optical coherence tomograph using spatially resolved Stokes parameters of the backscattered light from highly scattering biological samples.
Many surgical procedures use laser radiation to selectively photocoagulate subsurface chromophores in biological tissue in order to obtain a desired therapeutic effect. What is needed is the diagnostic imaging system to provide real-time feedback control for optical laser dosimetry. Application of surgical techniques using radiative energy to selectively destroy chromophores in tissue is limited by the availability of feedback techniques for real-time control of the laser dosimetry. A generic radiative treatment apparatus consists of a generic radiative treatment apparatus and is comprised of five major subsystems, namely (1) a radiative source such as a laser; (2) a diagnostic imaging subsystem; (3) image processing and analysis subsystem; (4) a feedback control subsystem; and (5) a delivering device which delivers the radiative energy to the sample.
Two non-contact methods have been proposed and tested as diagnostic imaging techniques for use in a generic radiative treatment apparatus. These methods include infrared imaging radiometry and polarization sensitive detection of the remitted backscattered light intensity. Both techniques are limited by poor spatial resolution of the inferred structural changes in the tissue which is induced by the incident radiation.
Infrared imaging radiometry uses a detector to measure temperature rises in a tissue induced by pulsed laser radiation. The temperature rise due to selective optical absorption of pulse laser light, creates an increase in infrared blackbody emission which is measured by the detector. Because biological materials are composed primarily of water, blackbody emission from tissue originates from a thin 5 to 50 .mu.m superficial layer on the tissue surface. Therefore, infrared imaging radiometry provides little useful information in the temperature distribution and result in structural changes at deep positions in the irradiated tissue. Furthermore, because the transport of thermal energy from deep portions to the tissue surface is a diffusive process, determination of underlying tissue changes is an ill-posed inverse problem that requires application of a computational algorithm to analyze the time sequence of infrared emission images. The ill-posed nature of the problem places severe limitations on the spatial resolution that can be realized from measurements through infrared imaging radiometry.
Polarization sensitive detection of remitted backscattered light intensity has recently been proposed as a non-contact imaging technique to monitor changes and tissue birefringence during laser irradiation. This technique relies on changes and the birefringence of native collagen in tissue upon exposure to laser irradiation. As collagen is subjected to higher temperatures over prolonged times, the molecular helix begins to unwind and birefringence is lost. If incident diagnostic light is linearly polarized, detected intensity changes of the remitted backscattered light from the tissue in the orthogonal polarization mode may indicate partial loss of collagen birefringence. Although this technique has been demonstrated to provide an average measure of bulk tissue birefringence changes, spatially resolved measurements have not been possible. Because this technique does not discriminate between the multitude of optical paths contained in remitted backscattered light from the tissue, spatial resolution in the lateral and longitudinal directions is limited by, respectively, the scattering anisotropy coefficient and the optical penetration depth. In the visible near infrared spectral regions, lateral and longitudinal spatial resolution is approximately 500 .mu.m.